Can a 3"5 hard drive use USB-C as its only power input?

Looking at some old 3.5" HDDs I have stacked in my basement I see that they require less than 1 amp on both the 5 volt and 12 volt DC input pins, most less than 1/2 an amp. This means they will take less than the 15 watts that many USB-C ports on many computers can supply, and many less than the 7.5 watts that is the minimum a USB-C port must provide to meet the USB-C spec.

A problem with this is that the ports on most computers will only supply 5 volts, and the drives will need 12 volts. It is possible to have a voltage boost circuit to create the 12 volt supply but that is an expensive circuit to build in the thin margins of consumer electronics. It is far less expensive to build a buck circuit to take a higher voltage to a lower voltage, and that's how some enclosures get the 3.3 volts that some drives use off a 5 volt supply from USB-C or USB-A.

It is unlikely such cases exist on the market. The profit margins are thin and the power budget is thin. The people making the cases must assume 5 volts at 1.5 amps from the USB-C port as that is the specified minimum for USB-C compliance. 3 amps from USB-C is common but not universal. 12 volts from USB-C was optional in USB-PD 1.0 but 5 years later became deprecated in favor of 9 volts and 15 volts when USB-PD 2.0 was published. USB-PD 1.0 was not very popular for some reason, it seems the people behind USB would rather everyone forgot it existed.

If we assume that the case for a 3.5" HDD must have room for 12 volts at 1 amp for the drive then that right there takes 12 watts. Then comes the 5 volt supply which might take 1/2 an amp, 2.5 watts. Then the interface board will need power. That's cutting it close even if the people engineering the case know that there will be 15 watts available.

The question did specify use of USB-PD for the drive enclosure power. USB-PD is only required if the client device is going to ask for more power than the 5 volts at 3 amps allowed with the USB 3.x spec. In most computers there will be a 12 volt supply but, again, supplying this out the USB-C port is not part of the current spec. It is allowed for backward compatibility to USB-PD 1.0 devices but, shall we say, "discouraged". Boosting that up to 15 volts is possible, then in the drive it can be bucked down to 12 volts and 5 volts easily enough. The USB-C ports on most computers that claim "high power" will be 15 watts (5 volts at 3 amps) or maybe 27 watts (9 volts at 3 amps). If they provide 12 volts then it's at most 27 watts, or 2.25 amps. Then after the 12 volts gets to the drive maybe there is more than 2 amps to work with, or maybe not because 2.25 amps is a maximum and not a minimum.

The thin power margins on USB-C under USB 3.x makes it difficult to power a 3.5" HDD. Requiring USB-PD for power means very few buyers as USB-PD output on computers is slim to none, and people willing to pay extra to not need an external power supply will also be slim to none. If such an enclosure exists then I'd be very surprised.

USB type is supposed to be able to supply 100w or 20v at 5amps.

It is physically possible to design a circuit to request the necessary power to provide all the power necessary to run a 3.5" hard drive.

1 amp at 5v is 5w and 1 amps at 12v is 12w. So the total is 17w. Now there will be a power loss in the conversion process, but if your USB type C port actually provides 100w there is a large margin of error.

However, the BIG problem is not every USB type C port is actually 100% compliant. Many don't offer the 100w option, but if yours does it is possible.

Now I don't know if anyone has actually done this or not.